Recently, as personal computers and television receivers have become lighter and thinner, reduction in thickness and weight of display devices has also been wanted. In answer to such demands, flat panel type displays such as liquid crystal displays (LCDs) have been developed in place of cathode ray tubes (CRTs).
An LCD is a display device which displays desired image data by applying electric fields across a liquid crystal layer having anisotropic dielectric constants, injected between a pair of substrates so that the strength of the electric fields is controlled to thereby control the amount of light passing through the substrates. Such LCDs are typical examples of handy flat panel type displays. Of these, TFT LCDs that employ thin-film transistors (TFT) as switching elements are mainly in use.
Lately, since LCDs have been not only used as the display devices of computers but also used widely as the display devices of television receivers, the need for rendering motion pictures has been increased. However, since the conventional LCDs are low in response speed, they have a drawback that it is difficult to reproduce motion pictures.
In order to make the LCD's response speed problem better, there is a known liquid crystal driving method wherein in accordance with the combination of the input image data of the previous frame and the input image data of the current frame, either a higher (overshot) drive voltage than the predetermined gray scale level voltage that corresponds to the input image data of the current frame or a lower (undershot) drive voltage is supplied to the liquid crystal display panel. In this specification of the present application, this driving scheme should be defined as overshoot (OS) drive.
FIG. 1 shows a schematic configuration of a conventional overshoot drive circuit. Specifically, the input image data (current data) of the N-th frame being about to be displayed and the input image data (previous data) of the (N−1)-th frame being stored in a frame memory 1 are loaded into an emphasis converter 2, wherein the patterns of the gray scale level transitions between both the data and the input image data of the N-th frame are looked up with the applied voltage data table stored in a table memory (ROM) 3 so as to identify applied voltage data, and write-gray scale level data (emphasis converted data) needed for image display of the N-th frame is determined based on the thus obtained applied voltage data (emphasis conversion parameters) so as to be supplied to a liquid crystal display panel 4. Here, emphasis converter 2 and table memory 3 constitute a write-gray scale level determining means.
The applied voltage data (emphasis conversion parameters) stored in the above table memory 3 is obtained beforehand from the actual measurement of the optical response characteristics of liquid crystal display panel 4. When, for example, the number of display signal levels, i.e., the number of display data, is 256 gray scales represented by 8 bits, every level of 256 gray scales may have a piece of applied voltage data. Alternatively, it is also possible that, as shown in FIG. 2, only the measurements for nine representative gray scale levels, one for every 32 gray scale levels, have been stored and the applied voltage data for other gray scale levels is determined by linear interpolation of the above measurements or other operations.
There has been a problem in that it takes long time to make a transition from a certain half gray scale level to another half gray scale level, so that it is impossible for a general liquid crystal display panel to display the half gray scales within the period of one frame (e.g., 16.7 msec. for a case of progressive scan of 60 Hz). This not only produces afterglow but also hinders correct half gray scale display. Use of the above-described overshoot drive circuit, however, enables display of the aimed half gray scale level within a short time as shown in FIG. 3.
As it has been also known that the response speed of liquid crystal greatly depends on the temperature, Japanese Patent Application Laid-open Hei 4 No.318516, for example, discloses a liquid crystal display panel driver that continuously controls and keeps the response speed of gray scale change in an optimal condition without loss of display quality in order to deal with any change of the temperature of liquid crystal display panel.
This configuration includes: RAM for storing one frame of digital image data for display; a temperature sensor for detecting the temperature of the liquid crystal display panel; and a data converting circuit which compares the aforementioned digital image data with the image data that is read out, by a one-frame delay, from the RAM and, if the current image data has changed from the image data one frame before, implements emphasis conversion of the current image data in the direction of the change, in accordance with the detected temperature of the above temperature sensor, whereby display of the liquid crystal display panel is driven based on the image data output from this data converting circuit.
Specifically, suppose that the temperature of the liquid crystal display panel to be detected by the temperature sensor is classified into, for example, three ranges Th, Tm and Tl (Th>Tm>Tl) and three mode signals, corresponding to these ranges, to be output from the A/D converter to the data converting circuit are defined as Mh, Mm and Ml, while in the ROM of the data converting circuit, “3”, the number equal to that of the mode signals, tables of image data, which can be accessed by designating the addresses or the value of the current image data and that of the image data delayed by one frame, are stored beforehand. One table which corresponds to the input mode signal is selected, and the image data stored in the table at the memory location designated by the addresses, i.e., the value of the current image data and that of the image data delayed by one frame is read out to be output to the drive circuit of the liquid crystal display panel.
However, in the above configuration disclosed in Japanese Patent Application Laid-open Hei 4 No.318516, three mode signals Mh, Mm and Ml are generated in accordance with the three range values Th, Tm and Tl (Th>Tm>Tl) for the detected temperature, and the emphasis conversion set of parameters is switched in accordance with the mode signal Mh, Mm or Ml. Therefore, if, for example, the detected temperature of the liquid crystal display panel is unstable and changes up and down over the ranges across Th, Tm and Tl, the mode signal also changes frequently between Mh, Mm and Ml, whereby for an identical gray scale level transition, the emphasis conversion parameter read from the ROM will vary. As a result, the image displayed on the liquid crystal display panel results in flickers and the like, degrading image quality.
Further, there are also cases where image quality is degraded due to temperature variation across liquid crystal display panel 4. For example, a rear view showing a schematic configuration of a liquid crystal display with a direct type backlight module is shown in FIG. 4. In FIG. 4, 4 designates a liquid crystal display panel, 11 fluorescent lamps for illuminating the liquid crystal display panel 4 from the rear, 12 an inverter transformer for energizing fluorescent lamps 11, 13 a power supply unit, 14 a video processing circuit board, 15 a sound processing circuit board and 16 a temperature sensor.
Of these, items releasing heat that greatly affects the response speed characteristic of liquid crystal display panel 4 are the electrode portions of fluorescent lamps 11, inverter transformer 12 and power supply unit 13. It is preferred that temperature sensor 16 is arranged inside liquid crystal display panel 4, from its due objective, but is difficult, so the sensor should be attached to another member such as a circuit board.
Therefore, when, for example, the constituents 11 to 15 are arranged as shown in FIG. 4, temperature sensor 16 is attached to sound processing circuit board 15, which is least affected by generation of heat from inverter transformer 12 and power supply unit 13, and the detected output from this temperature sensor 16 is made use of by an overshoot drive circuit provided in video processing circuit board 14.
Also as in a liquid crystal display of a direct type backlight using U-shaped fluorescent lamps 11 shown in FIG. 5(a), in a liquid crystal display of a side-edge type backlight using L-shaped fluorescent lamps 11 shown in FIG. 5(b) or in any other like configuration, large temperature rises occur in the partial areas of liquid crystal display panel 4 where the electrode portions of fluorescent lamps 11 and the inverter transformer for energizing fluorescent lamps 11 are arranged, so other areas increase more in temperature compared to the hatched areas in FIG. 5.
Here, in the conventional liquid crystal displays, the detected temperature by a single temperature sensor 16 is assumed to be the temperature of the whole of liquid crystal display panel 4 and overshoot drive control is implemented every frame based on this detection. In the actual situation, however, liquid crystal display panel 4 has varying temperature distribution across the panel surface depending on the arrangements of the heat generating elements as stated above.
Resultantly, in the partial areas on liquid crystal display panel 4 where temperature is higher than the detected temperature of temperature sensor 16, insufficient applied voltages of data (emphasis converted data) are supplied possibly causing shadow tailing. On the other hand, in the partial areas on liquid crystal display panel 4 where temperature is lower than the detected temperature of temperature sensor 16, excessive applied voltages of data (emphasis converted data) are applied possibly causing white spots and the like (when in the normally black mode), causing significant image quality degradation of the display image.
Similarly, if this liquid crystal display is put in a place where air is blown onto it from a room air-conditioner or in a sunny place or direct sunshine, part of liquid crystal display panel 4 may decrease or increase in temperature, producing varying temperature distribution across the surface of liquid crystal display panel 4. Resultantly, excessive applied voltages of data (emphasis converted data) may be supplied in partial areas, producing white spots, or insufficient applied voltages of data (emphasis converted data) may be supplied to liquid crystal display panel 4 causing shadow tailing (when in the normally black mode), hence image quality of the display image is significantly degraded. This problem of varying temperature distribution across liquid crystal display panel 4 depending on the place of installation becomes more noticeable as the display screen size becomes greater.
Further, in the case of the above-described conventional liquid crystal display, in the normal installed state (stand-mounted state) shown in FIG. 6(a) temperature sensor 16 is arranged at the place where it has least influence of heat from inverter transformer 12, power supply unit 13 and other components. However, when the screen is set at the vertically inverted state (in the suspended state from ceiling) as shown in FIG. 6(b) or when rotated by 90 degrees (in the portrait orientation mode) as shown in FIG. 6(c), the heat flow path changes hence temperature sensor 16 is significantly affected by generation of heat from the other members, so it is no longer possible to detect the exact temperature of liquid crystal display panel 4.
As a result, correct applied voltages of data (emphasis converted data) corresponding to the temperature of liquid crystal display panel 4 cannot be supplied to liquid crystal display panel 4, causing the problem of image quality of the display image being significantly degraded by shadow tailing due to application of insufficient applied voltages of data (emphasis converted data) to liquid crystal display panel 4 or by white spots due to application of excessive applied voltages of data (emphasis converted data) to liquid crystal display panel 4, (in the case of the normally black mode).
In view of the above, the present invention has been devised to solve the above problem, it is therefore an object to provide a liquid crystal display which can improve the image quality of the display image by making variable control of emphasis conversion parameters in a stable manner even if the detected temperature of the device interior is unstable.
It is another object to provide a liquid crystal display which can prevent image degradation of the display image even if varying temperature distribution across the screen surface of the liquid crystal display panel takes place.